The present invention relates broadly to an electronic liquid flow meter and, in particular to a liquid flow meter for domestic and commercial use.
Traditional liquid flow meters in domestic and commercial use generally include some mechanical arrangement such as a bellows, or a vane or impeller which actuates a totalising mechanism. Generally, a mechanical dial arrangement indicates the total volume of liquid that has passed through a meter Such mechanical arrangements are not highly accurate, especially at low flow rates. Accordingly, inaccuracies in low flow rate measurement can represent a substantial loss of income to the supplier of water or other liquids such as hydrocarbons.
Over recent years, there have been a number of proposals that utilise electronics technology so as to provide for substantial higher accuracy of the fluid flow measurement. Such systems generally incorporate ultrasonic transducers that transmit ultrasonic signals both upstream and downstream to measure the times of flight of the signals from which the relative speed of the fluid can be calculated. A further method measures the phase change between two signals which are simultaneously transmitted from the transducers in order to calculate the speed of the fluid.
However, problems arise with the use of ultrasonic signals due to substantial variations in amplitude and phase of the waves transmitted and received by the ultrasonic transducer, resulting in inaccurate measurements. These variations can arise due to changes in temperature, a build-up of material on the transducer heads which affects impedance matching of circuit components, and also ageing and micro-cracking of the transduction elements.
Variations must be allowed for during the design and calibration of transducer circuits with the xe2x80x9creciprocity theoremxe2x80x9d being applied to the circuit components. The Chambers Dictionary of Science and Technology (1991) defines the xe2x80x9creciprocity theoremxe2x80x9d as xe2x80x9cthe interchange of electronic force at any one point in a network and the current produced at any other point results in the same current for the same electromotive forcexe2x80x9d. In application to acoustics, the theorem essentially says that a transmitter and a receiver may be swapped to give a reciprocal electro-acoustic transformation Existing arrangements fail to achieve true reciprocity in ultrasonic transducer calibrations. Thus, accurate measurement, particularly at very low flow rates has been unable to be achieved.
It is an object of the present invention to substantially overcome, or ameliorate, one or more of the deficiencies of the above mentioned arrangement by provision of a liquid flow meter that is accurate over a wide range of temperatures and operating conditions.
According to one aspect of the present invention there is disclosed a liquid flow meter for directly measuring the velocity of a liquid, said liquid flow meter including:
a pair of transducers arranged facing each other in a conduit through which the liquid flows;
transmitter means for causing the transducers to simultaneously transmit an acoustic wave packet directed for reception at the other said transducer;
differential receiver means having inputs each coupled to a corresponding one of said transducers for detecting an acoustic signal received thereby and determining a difference between the two received signals, said difference being related to the velocity of liquid within the conduit, wherein the transmitter means and said differential receiver means are each matched to said transducers to ensure substantial reciprocity to thereby substantially avoid phase and/or amplitude variations in said received signals.
The acoustic wave packet as transmitted preferably comprises a predetermined plurality of cycles.
Preferably the differential receiver means comprises a pair of receiver amplifiers each coupled to a corresponding one of the transducers and outputting the respective inputs of a differential detector. In a specific embodiment the differential detector is formed by a transformer having the terminals of a primary winding coupled to the respective outputs of the receiver amplifiers.
The differential detector preferably outputs a difference waveform, wherein the difference waveform is related to the velocity of the liquid within the conduit.
The liquid flow meter preferably includes a processing means, wherein the processing means removes noise from the difference waveform and calculates the difference between the two received signals.
The processing means farther preferably produces a sinusoidal pulse train at a predetermined frequency which is used to electrically excite the liquid flow meter. Preferably the predetermined frequency is about 1 MHz.
According to another aspect of the present invention there is provided a A method for directly measuring the velocity of a liquid, said method comprising the steps of:
simultaneously transmitting an acoustic wave packet between a pair of transducers arranged facing each other in a conduit through which said liquid flows;
a detecting an acoustic signal received by differential receiver means having inputs each coupled to a corresponding one of said transducers; and
determining a difference between the two received signals, said difference being related to the velocity of liquid within the conduit, wherein the transmitter means and said differential receiver means are each matched to said transducers to ensure substantial reciprocity to thereby substantially avoid phase and/or amplitude variations in said received signals.